Setting up a quarantine system is your best bet in future. Can't think of any store near me that has a proper quarantine system, every store I've seen dumps their fish straight from the distributor into their display tanks. If they spot any issues they'll quarantine that particular tank after the fact. Some do take a bit more care with their handling and all that but there's just too much volume and not enough floor space in most shops to have dedicated quarantine. Grab the kmart special if you're keeping small fish, fill the filter with some sponge from your existing setup whenever you do want/need to quarantine.